The passive transfer of convalescent sera did not protect the majority of mice against challenge with the homologous strain and was completely ineffective against challenge with strains unrelated by fluorescent antibody techniques. When the immune sera was incubated with the rickettsia in vitro and then inoculated into the mice a dramatic increase occurred in the number of surviving mice. The importance of these data in relation to published results with other species of rickettsia is discussed.
The overall comparisons of habitats are given in (Table III). The habitats are arranged in order of extent of alterations by man, with the least disturbed at the top. The highest average blood isolation rates came from the least disturbed areas. The highest monthly maximal rickettsial isolation rates from blood and maximal prevalence rates of antibody per month were also obtained at Bukit Lanjan, the habitat least altered by activities of man. The lowest average blood isolation rate (6%) and the lowest monthly maximal rickettsial isolation and antibody prevalence rates were obtained at Bukit Mandol, the habitat most extensively and intensively altered by man. The intermediate habitats had intermediate rates. We caution anyone interpreting these observations, however, in terms of human disease, which seem to be associated with hyperendemic foci. Here we are not dealing with hyperendemicity from the standpoint of human disease, but present evidence of widespread endemicity from which hyperendemic foci may derive. Also, we have not yet identified the prevalent strains and do not know their infectivity to man.
Based on the prevalence of antibody, an estimated 3% of the population of rural Malaysia is infected with Rickettsia tsutsugamushi each year, resulting in positive antibody rates in focal areas of 6 to 69%. Most of these infections do not appear to produce clinical scrub typhus. A wide range of seropositivity rates was found in areas otherwise resembling each other in predominant occupation, terrain, and nearby habitat. The prevalence rates however were significantly higher in people who worked in forested areas and significantly lower in people with urban occupations.
An explanation was sought for the disparity between the low reported incidence of scrub typhus and the high prevalence of antibody to Rickettsia tsutsugamushi in the rural population of Malaysia. A combination of isolation of the organism, titration of antibody by indirect immunofluorescence, and the Weil-Felix test was used to confirm infections. Scrub typhus was found to be very common, causing 23% of all febrile illnesses at one hospital. The infection was particularly prevalent in oil-palm workers, causing an estimated 400 cases annually in a population of 10,000 people living on one plantation. The clinical syndrome, whether mild or severe, was difficult to distinguish from that due to other infections. Eschars, rashes and adenopathy were uncommon. When used to examine early sera, the Weil-Felix test failed to confirm the diagnosis in most infections.20
Leptotrombidium (Leptotrombidium) arenicola Traub, a vector of scrub typhus, had previously been found to occur in the coastal vegetation behind the edge of open sand along the beaches of Peninsular Malaysia. Surveys of the west coast beaches of Sabah and Sarawak were conducted to determine if this species occurs in similar habitat in East Malaysia on the island of Borneo. Leptotrombidium (L.) arenicola was not collected from the eighteen sites studied. Of the 11,982 mite larvae collected, 55 per cent were L.(L.) deliense (Walch), a well-known, widespread vector of scrub typhus.
The life cycle of 5 generations of Leptotrombidium (L.) fletcheri infected with Rickettsia tsutsugamushi and reared under ambient temperatures in Malaysia was presented and compared with a colony reared at a constant 27 degrees C (Neal and Barnett, 1961). In general our colony had a longer generation time (average of 54 days from engorged larvae to adult compared with 37 days) and produced fewer eggs (average of 127.9 compared with 900.0) than the comparison colony. Possible factors causing these differences are discussed.
Dog sera, collected from different communities throughout Selangor, Peninsular Malaysia, were investigated for the presence of antibodies to R. tsutsugamushi and R. typhi. Scrub typhus antibodies were present in animals from the rural areas only, whereas murine typhus antibodies were observed in equal numbers of dogs from both rural and metropolitan areas. Greater percentage of dogs from suburban areas had demonstrable antibody titers to murine typhus than from the urban area.
Rickettsia tsutsugamushi isolations were attempted from blood samples obtained from rats captured in four adjacent habitats near Kuala Lumpur, Malaysia. Antibody surveys were also made. Rickettsial infections were most frequent in rats captured in the forest and in lalang grass (Imperata cylindrica) and least frequent in the most extensively disturbed habitat, an Orang Asli (aborigine) village. Forest rats such as Rattus sabanus (31%), as well as rats in the subgenus R. (Rattus), i.e. R. tiomanicus (26%) and R. argentiventer (35%) had frequent active infections. The house rat R. exulans had less frequent infections (15%). Frequency of antibody occurrence followed a similar pattern. No marked seasonal differences in the frequency of infections could be detected during the 18-month study.
Two communities of Orang Asli (aborigines) in Peninsular Malaysia were observed for evidence of Rickettsia tsutsugamushi infection over periods of 1-8 mo. Sequential sera were examined for antibody by the indirect immunofluorescence test. The incidence of infection in the two self-selected populations in the two communities was calculated to be 3.9% per month and 3.2% per month.
An epidemiological study in a mature oil palm estate in Peninsular Malaysia has demonstrated a low prevalence of R. tsutsugamushi infection in small mammals. The direct fluorescent antibody technique for assaying infections in chiggers proved more sensitive than mouse inoculation. Most infections in both chiggers and rodents were caused by the Karp strain.
One hundred and fourteen Rickettsia tsutsugamushi isolates, recovered from febrile patients in central Peninsular Malaysia, were antigenically analyzed by direct immunofluorescence using eight prototype strains. Twenty-nine antigenic types were detected. The TA763, TA716, Karp and TA686 strains were the most common and occurred singly or in combination with each other or other strains in 86% of the isolates.
Fifty-one Rickettsia tsutsugamushi isolates from small mammals collected in central Peninsular Malaysia serologically characterized by direct immunofluorescence using eight prototype strains. Karp-related (TA763, TA686, TA716) antigens were found in 90.2% of the isolates.
Serological surveillance for up to two years of 114 patients with laboratory confirmed scrub typhus showed that antibody to Rickettsia tsutsugamushi as demonstrated by the indirect fluorescent antibody test is short-lived. The mean reversion time from mean peak titre (1:499) was 48.9 weeks and the calculated annual reversion rate to a titre less than 1:50 was 61%. This can be used to estimate attack rates based on point prevalence of antibody. The relationship between antibody prevalence and attack rates observed by other workers was confirmed using this model. The possible uses of the finding and its implications in Malaysia are briefly discussed.
Scrub typhus is a widespread and at times serious infection in Asia. If results from central Malaysia can be applied, it appears to be economically important. Diagnosis is often difficult and treatment prone to fail if short courses of antibiotics are used. Prophylaxis is the key area of research with the development of a vaccine being the ultimate goal.
Using an indirect immunofluorescence technique, sera from 113 cynomolgus monkeys (Macaca fascicularis), trapped in Peninsular Malaysia, were screened for the presence of antibody to six prototype strains of Rickettsia tsutsugamushi combined into three polyvalent groupings: I--Karp, TA716, and TA763; II--Gilliam; and III--TA678 and TH1817. Fifteen percent (17/113) of the monkeys had antibody titers greater than or equal to 1:50 to one or more of the antigenic groups. Although a titer greater than or equal to 1:150 is generally considered indicative or prior Rickettsia tsutsugamushi infection, we selected a less than 1:25 titer as a conservative standard to insure non-infected animals. Using this criterion, 62 (55%) of the 113 monkeys were accepted for use in scrub typhus studies. The high prevalence of antibody to scrub typhus in the semi-arboreal cynomolgus monkey is in marked contrast to the low prevalence reported in the strictly arboreal silvered leaf monkeys (Presbytis cristatus). The results of this study indicate that cynomolgus monkeys should be rigorously screened for evidence of prior infection before they are included in experimental scrub typhus studies.